Personal Entries

Physics question for you

My little blogosphere nerds! I know you’re out there, and it’s time to step up.

While playing Scrabble today, Matt asked this question:

“If someone were falling, and someone else shoved them as they fell, would it slow them down?”

Karen and I thought that, yes, it would slow them down.  I added that how much it slowed them down would depend on how far they had fallen when the shove occurred, and the total height of the fall after the shove (for instance, if there was enough distance left to fall that they were able to regain their original falling speed before impact).

 

A physics problem, which is a VERY NORMAL thing for me to diagram. I am CONSTANTLY diagramming out physics problems. * Object in right-most scenario is a tree.

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And because diagrams ALWAYS help, I quickly drew one on our Scrabble tally sheet.  Karen commented that drawing a diagram about a physics problem was a switch, as I usually only diagram out issues of reproduction, fertility, and breasts.  Karen is often wrong, as I’ve noted on the diagram: Scrabble tallies are highlighted in blue with the final scores starred in green.  I totally beat her because she is so wrong, and I diagram much, much more than the sexy things.

I would call Jake at Chapel Hill to ask him to answer Matt’s question, but I’m a bit annoyed with Jake today.  Go ahead, ASK ME WHY and I will totally tell you.

But, blogosphere brainiacs, are we right? Would a mid-fall shove slow the faller down?  Would the amount by which the fall was slowed depend on at what point the shove occurred; how much fall was left?

Help me, Obi Wans.

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*Bethany’s comment made me rethink my phrasing, so let me rephrase:

Would the shove slow the momentum of the fall? When you fall, I assume it takes a certain amount of time to get up to top speed. The shove would slow that speed, right? So that you wouldn’t hit the ground at the same speed that you would have had you not been shoved?

For example, if you fell off a skyscraper, and someone/something shoved you BUT GOOD about 15 feet from the ground, would you hit the ground less hard than had you not been shoved?


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7 thoughts on “Physics question for you

  1. Well the shortest distance between two locations is a straight line. Falling with no shove is a close as you’re gonna get to a straight line. So I’d think if you wavered from that path (ie, a shove.) that it would slow you down. Probably not significantly unless it was a really hard shove. But it would slow you down.

    My opinion is purely logical and not at all scientific, because I never took physics and I be stupid.

    1. I hadn’t thought of that: that you’re increasing the distance of the fall, and it will therefore take longer to fall.

      You totally got my brain going, so I probably shouldn’t have checked my blog comments when I got up in the middle of the night to eat cookies. It leaves me thinking about physics at 2:29 am.

      Alright….Let me rephrase my question……would the shove slow the momentum of the fall? When you fall, I assume it takes a certain amount of time to get up to top speed. The shove would slow that speed, right? So that you wouldn’t hit the ground at the same speed that you would have had you not been shoved?

      For example, if you fell off a skyscraper, and someone/something shoved you BUT GOOD about 15 feet from the ground, would you hit the ground less hard than had you not been shoved?

      1. Assuming that the person is falling towards the earth, if they were shoved in the direction of the earth (i.e.) down, then this would
        1. Increase the speed at which they are falling, since you are adding an instantaneous force, and an increase in acceleration causes an increase in velocity.
        2. Increase the momentum since momentum = speed*mass, and they certainly aren’t gaining any mass by being pushed.

        It is possible if the person were pushed while they had reached terminal velocity, that they would increase for a brief period of time, then decrease back to the terminal velocity. Terminal velocity has to do with the opposing wind speed pushing in the opposite direction of gravity.

        They would absolutely not decrease in speed though, unless for a brief moment under the terminal velocity assumption.

        Force = mass*acceleration.

        (no-pushing case)
        Force = mass*(acceleration_gravity)

        (pushing case)
        Force = mass*(acceleration_gravity) + Force_push

        Since Force_push is in the same direction as the acceleration to due gravity,

        Force pushing > Force non-pushing

        1. What if the fall were towards the earth, but the shove was from the side? Using the numbers on a clock as an example of direction: The fall originates at 12, and is headed to 6. The shove comes from the direction of 9, shoving towards 3?

          1. That would contribute an instantaneous horizontal velocity component to the person, but horizontal acceleration would remain zero unless they continued to be pushed for more than an instant. The person would fall in a parabolic path, rather than straight down.

            Velocity_total = sqrt (velocity_horz^2 + velocity_vertical^2) >= velocity_veritcal.

            (i.e. the velocity would still be greater)

            The only way to slow the person down is to push them in the opposite direction of the force of gravity.

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